The present invention relates to a method of storing trichlorosilane and silicon tetrachloride. More specifically, it relates to a method of storing trichlorosilane and silicon tetrachloride for the production of high-purity polycrystal silicon which is a raw material of silicon for devices.
The most general method of producing high-purity polycrystal silicon uses trichlorosilane as a raw material. Trichlorosilane is first purified by distillation and separated into high-purity trichlorosilane, other chlorosilanes and impurities to be discarded. Thereafter, the purified trichlorosilane obtained by distillation is reduced by reacting it with hydrogen to produce high-purity polycrystal silicon. Particularly, a method of producing a rod-shaped deposit is called "Siemens method" and widely and commonly employed.
Since the above purified trichlorosilane has a low boiling point and readily reacts with moisture contained in the air, it is extremely dangerous and hence, stored in a pressure-resistant tank. When it contacts the air, it reacts with oxygen in the air and may ignite. Therefore, its storage tank must prevent it from contacting the air by sealing with a gas which is virtually inactive with trichlorosilane. A nitrogen gas has been used as a means for preventing trichlorosilane from contacting the air (to be simply referred to as "sealing means" hereinafter).
It has been believed so far that nitrogen is inactive with trichlorosilane. The inventors of the present invention investigated the influence of nitrogen on the whole production process of polycrystal silicon. Surprisingly, they have found the following two facts.
One of the facts is that nitrogen does not react with trichlorosilane directly, but it forms a nitride when it is hot, for example, when supplied to a precipitation reaction. To precipitate polycrystal silicon, hydrogen and gasified purified trichlorosilane are used. The surface of a polycrystal silicon rod produced by supplying these gases has a fair gray color in a normal state. The present inventor has conducted experiments in which after polycrystal silicon was precipitated through the reaction of trichlorosilane with hydrogen, the supply of trichlorosilane was stopped and nitrogen was supplied. At this point, a current for heating polycrystal silicon was kept applied. After nitrogen was supplied for about 1 minute, a current was stopped as usual and polycrystal silicon was cooled and taken out. The surface of the rod changed its color to black by a nitride.
The other fact is that nitrogen, unexpectedly, dissolves in trichlorosilane extremely easily. The experiments revealed that about 2,300 ppm (molar fraction) of nitrogen dissolves in trichlorosilane at normal temperature. This figure is hardly conceivable from common knowledge, but has been verified from both aspects by measurement by gas chromatography and checking of the mass balance of nitrogen in a polycrystal silicon production plant.
By discovering the above two facts, a reduction in the quality of polycrystal silicon caused by the following has been concerned. In an attempt to store trichlorosilane, if the above-described means of preventing it from contacting with the air by charging nitrogen, the charged nitrogen dissolves in trichlorosilane. When the trichlorosilane is used to precipitate polycrystal silicon, the above nitride is formed inside the polycrystal silicon rod. Further, hydrogen is used in a precipitation reaction to increase the productivity of silicon. In general, this hydrogen is mixed with trichlorosilane before it enters a reactor. Chlorosilane and hydrogen chloride are separated from the exhaust gas from the reactor and this hydrogen is recycled. When nitrogen released from trichlorosilane is mixed in the reactor, nitrogen mixed into the hydrogen is accumulated in a hydrogen recycling line. As a result, the concentration of nitrogen contained in the gas to be supplied to the reactor increases gradually.
Since a nitride contained in polycrystal silicon which is precipitated using hydrogen and trichlorosilane containing nitrogen has a low concentration, it cannot be observed directly by currently known analytical means. Meanwhile, although the cause is not yet clearly known, it has been known that a reduction in the production yield of devices by the occurrence of OSF (Oxidation-Induced Stacking Fault) or the like is influenced by a process for producing polycrystal silicon. For example, as disclosed in JP-A Hei 3-80193 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), it is often said that a heavy metal causes it, but it is conceivable that the heavy metal, as a matter of fact, is not solely responsible for it. It is conceivable that a silicon nitride remains as a solid matter when polycrystal silicon is pulled up as monocrystals due to an extremely high melting point of a silicon nitride. Although it cannot be determined based on the above fact whether a nitride influences the yield, it can be at least said that there is a great possibility that the nitride is one of the primary factors.
It is an object of the present invention to provide a method of storing trichlorosilane and silicon tetrachloride using a hydrogen gas as a sealing gas.
It is another object of the present invention to provide a method of storing trichlorosilane which is advantageously used in a process for producing high-quality polycrystal silicon using trichlorosilane as a silicon source.
It is still another object of the present invention to provide a method of storing trichlorosilane which inhibits the formation of a nitride in polycrystal silicon to provide high-quality polycrystal silicon as a raw material for high-density ICs whose yield is expected to be increased at the end.
It is still another object of the present invention to provide an industrially highly valued process for producing polycrystal silicon by reacting trichlorosilane with hydrogen, which comprises the steps of storing trichlorosilane as a raw material and/or silicon tetrachloride by-produced in the above reaction, using a hydrogen gas as a sealing gas, and converting the silicon tetrachloride into trichlorosilane.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention can be attained by a method of storing trichlorosilane using a hydrogen gas as a sealing gas for a trichlorosilane storage tank.
Secondly, the above objects and advantages of the present invention can be attained by a method of storing silicone tetrachloride using a hydrogen gas as a sealing gas for a silicon tetrachloride storage tank.
Thirdly, the above objects and advantages of the present invention can be attained by a process for producing polycrystal silicon comprising the step (1) of forming polycrystal silicon by reacting trichlorosilane with hydrogen, the step (2) of forming trichlorosilane by reacting silicon tetrachloride, by-produced in the polycrystal silicon-forming step (1), with hydrogen and metal silicon, and the step of supplying trichlorosilane, containing at least part of the trichlorosilane obtained in the trichlorosilane-forming step (2), to the polycrystal silicon forming step, wherein trichlorosilane used in the polycrystal silicon-forming step (1) and/or silicon tetrachloride used in the trichlorosilane-forming step (2) are stored in storage tank(s) using a hydrogen as a sealing gas and supplied to the respective steps from the tank(s).